Homebrew Castable 21" Kent (UK)

[bamforp]

After another hiatus, I am back working on my son's oven. First of all apologies to Marko for not responding to your post - I somehow missed the notification email but hopefully this update will answer your question.

My daughter-in-law made a nice job of adding mosaic tiles to the arch using some heat resistant tile adhesive and I set the arch on a bed of mortar whilst holding it in place temporarily with string. I fitted 2 x 25mm layers of ceramic blanket to the dome (reduced to 1x 25mm around the gallery) and fixed the stainless steel flue pipe with a 5:1 vermicrete mix which I extended up the pipe maybe 150mm. All very straightforward so far and ready for the vermicrete outer layer.

[bamforp]

Okay - I found this to be the most challenging part of the project so far. I have not worked with vermicrete before so I did not really know what to expect and it ended up being a bit of a wrestling match between me and gravity! The last photo shows the kind of aesthetic I am aiming for albeit on a much smaller scale - I really enjoy the Wood Fired Oven Chef's YouTube channel btw and I recommend checking it out if you have not seen it.

I found hand mixing a 10:1 vermicrete in roughly 6L batches worked best for me (with a little fireclay per batch) and it went on easily over the top of the dome but the sides were a different matter. Anything other than a thin (maybe 15mm) layer just fell off and I realised I was going to need to some sort of formwork for the sides. The second photo shows where I'd got to after several hours messing about and a lot of cursing. The next day, I cobbled together some formwork using carboard which worked reasonably well and I added more vermicrete to get to roughly the shape I was aiming for and left it to harden overnight. The next day I gently removed the formwork fearing the worst but in fact most of the vermicrete stayed in place apart from some areas at the sides (see photo 4) which I had to patch. These were the thinnest parts of the vermicrete layer and I also think it did not bond very well to the layer I had applied the previous day. Anyway, with a bit more fettling I managed to get reasonably close to the desired shape and it's now wrapped up in a tarp to fully cure for the next few days.

In hindsight, I should have had the formwork in place from the start and 'cast' the vermicrete in one go and I think it would have been a much more straightforward job. Anyway, I am planning to go back in a few days and hopefully we will be ready to start the drying out process before applying the final render coat.

[david s]

That’s a terrific result.
Applying a lean vermicrete brew is difficult. For future reference the finer grade is more workable and the correct water addition is critical. I suspect you may have either used a course grade and or either too much or too little water. I find a 10:1 mix about as lean as is comfortable to apply, but some go as lean as 13:1 for better insulation value.

[bamforp]

Thanks David. The grade of vermiculite that is readily available at a sensible price in the UK is fairly coarse - it is intended for jobs like backfilling between a flue liner and the chimney brickwork. I think finer horticultural grades are available via garden centres but the price is horrendous. I did start out with a 3 parts water recipe but ended up just adjusting the water mix by what looked about right, partly because some of the vermiculite was a bit damp from having been stored outside for a while in leaky bags.

Does the vermicrete normally set solid? After a day, mine was firm but not very strong - I think it should be okay to render as long as I don't press too hard with the trowel.

[david s]

Yes, you just want it firm enough to render against.

[Warhorn6]

Looking great so far. Any progress? I can only hope my oven turns out half as good as what your looks like.

[Ronstarch]

Very late to the party here, but I’ll add my own tip on applying vermicrete / perlcrete:

Don’t try to apply it to the sides of your oven like a stucco or render. Instead, build it up in layers (courses) starting from the bottom. Each layer rests on top of the previous layer. This way gravity is helping, not hurting. Basically you’re building a vermicrete shell around your oven.

thanks for the write up on your build, it looks great, hope it makes good pizzas for you!

[bamforp]

After what seemed like a never ending winter I am back to work on my project. Despite being under a tarp, the vermicrete took a bit of a beating over the winter but once I brushed off the flaky bits the main structure was sound. I used a ready mixed one coat render which was very easy to use although it is very aggressive on the skin and I should have used gloves. The first step was a rough coat to bring all the surfaces onto the same 'plane' so that a top coat would go on easily. It took me a full day and 4.5 bags of render to complete but I was pretty happy with the end result so I rewarded myself with a beer! I used a bit of cardboard to leave a gap between the steel chimney liner and the render which I will stuff with some fire rope and seal with some high temp silicone.

I have been spraying it with water every few hours to try and retard the curing and the plan is to paint it with masonry paint once it is fully dry.

[bamforp]

Over the winter I had made the bits for a door using some plywood and an aerated concrete block cut to shape with a jig saw. I assembled the door with a few blobs of high temp silicone and a couple of steel brackets to protect the base. A bit of fire rope around the edge seals it against the mouth of the oven.

Although we have not made pizza yet, we have been lighting a few fires to gradually dry out the oven. It was quite hard to control the temperatures and a couple of hairline cracks have appeared. One at the back of the oven near the centre although interestingly it has not propagated beyond the break I introduced in the 'cap' of the dome. The other crack is at the mouth of the oven but neither of the cracks are causing me any concern at this stage and have not impacted the integrity of the dome at all.

Hopefully the next update will be me enjoying fresh pizza and (another) beer.

[david s]

After what seemed like a never ending winter I am back to work on my project. Despite being under a tarp, the vermicrete took a bit of a beating over the winter but once I brushed off the flaky bits the main structure was sound. I used a ready mixed one coat render which was very easy to use although it is very aggressive on the skin and I should have used gloves. The first step was a rough coat to bring all the surfaces onto the same 'plane' so that a top coat would go on easily. It took me a full day and 4.5 bags of render to complete but I was pretty happy with the end result so I rewarded myself with a beer! I used a bit of cardboard to leave a gap between the steel chimney liner and the render which I will stuff with some fire rope and seal with some high temp silicone.

I have been spraying it with water every few hours to try and retard the curing and the plan is to paint it with masonry paint once it is fully dry.

Retaining the moisture in anything made with Portland cement is vital to enhance its strength. Spraying every few hours leads to a wet and dry cycle which, although better than nor doing it, is not as good as holding it continuously. Wrapping the whole thing in cling wrap for a week works pretty well. This is important because applying a reminder over a vermicrete or perlite substrate will tend to suck some moisture away. Preventing evaporation from the outer surface then becomes even more essential.

"If enough water evaporates from the concrete before it has attained its maximum strength, there will not be sufficient water remaining in the concrete to fully hydrate the cement and so achieve that maximum strength."
Here's a fuller explanation.

https://www.holcim.com.au/products-a...ete-techniques

[bamforp]

That's a good steer David - thanks

[Kris S]

Your oven looks amazing! I really like the shape of the wood fired chef's oven.

Wish you'd built yours before mine, than I could've modelled mine on yours!

[bamforp]

Thanks Kris - here's hoping it makes good pizza's.

[Ash75]

This is amazing build Peter. Your photos through various states tell the story. Do you have a list of materials and supplies as well as cost? I am debating between homebrew cast oven or refractory cast oven. However your project is very inspiring. I am in Manchester and would love to know your suppliers. Ash

[UtahBeehiver]

Look at Nick JC's build, it was a nicely done cast oven, he is from the UK and he listed all his suppliers in the thread.